1. Field of the Invention
The present invention relates to a process for producing oxamide (oxalic acid diamide) granules. More particularly, the present invention relates to a process for industrially producing oxamide granules with a high degree of purity by a series of operations including the steps of reacting an oxalic acid diester with ammonia, granulating the reaction product and drying the resultant granules within the same one apparatus.
The oxamide granules produced by the process of the present invention are practically usable as an extremely beneficial slow release fertilizer.
2. Description of the Related Arts
As a typical process for producing oxamide Japanese Unexamined Patent Publication No. 52-7,916 discloses a process comprising the steps of mixing an aliphatic alcohol with oxalic acid diester under specific conditions, then adding ammonia to the mixture to cause the oxalic acid diester to react with ammonia and produce oxamide.
In the above-mentioned process, the production of oxamide from an oxalic acid diester with ammonia is carried out in accordance with the following chemical reaction: EQU (COOR).sub.2 +2NH.sub.3 .fwdarw.(CONH.sub.2).sub.2 +2ROH
wherein R represents a lower alkyl group having 1 to 6 carbon atoms, for example, a methyl, ethyl, propyl, butyl or pentyl group.
In the above-mentioned conventional process, the reaction of an oxalic acid diester with ammonia is carried out in a solution system containing an excessive amount of an aliphatic alcohol. This excessive amount of aliphatic alcohol exhibits an effect of restricting the production of a by-product consisting of oxalic acid monoestermonoamide during the above-mentioned reaction procedure, and therefore, the conventional process can produce oxamide with a high degree of purity at a high yield.
As mentioned above, the production of oxamide is carried out in a liquid phase reaction system which always contains an excessive amount of an aliphatic alcohol. Also, the resultant oxamide is significantly insoluble in the aliphatic alcohol. Therefore, when the oxamide-producing reaction is completed, the resultant reaction mixture is in the state of a slurry of a powdery oxamide. To obtain a high purity oxamide, it is necessary to collect the powdery oxamide from the slurry, by filtration, and remove the aliphatic alcohol contained in the collected powdery oxamide by a drying procedure at a high temperature. Namely, the conventional process for producing oxamide is disadvantageous in that the resultant oxamide must be purified by complicated refining procedures and apparatuses, and a large amount of heat energy must be consumed by these refining procedures.
In a previous invention by the inventors of the present invention, a process for producing an oxamide powder was provided. In this process, ammonia is feed-mixed to a melt of an oxalic acid diester to produce a solid mixture containing the resultant oxamide, and the resultant solid mixture is heated to evaporate-remove a by-product consisting of an aliphatic alcohol to provide an oxamide powder.
In order to use the oxamide powder as a slow action fertilizer, it is necessary to granulate the oxamide powder into a plurality of granules by using a granulating apparatus. Nevertheless, oxamide is not soluble in all the solvents including water, and thus exhibits a very poor ability to be granulated. Therefore, when oxamide is granulated by a conventional granulating apparatus as disclosed, for example, by Japanese Unexamined Patent Publication (Kokai) No. 59-169,527, it is necessary to use an expensive binder, for example, polyvinyl alcohol or to consume a large amount of energy. Otherwise a special granulating method, as shown in Japanese Unexamined Patent Publication No 57-160,985 for example, a compaction granulating method under a high pressure, which has a very poor efficiency, must be employed. Also, those conventional granulating methods and apparatuses are very complicated, and require a high processing cost and expensive equipment.
Further, there is a strong demand to provide oxamide granules having a satisfactory mechanical strength and form-stability in water, and thus easily usable as a slow action fertilizer.